This invention relates to a modem for use in a spread spectrum (often abbreviated in the art to SS) multiplex communication network comprising a directional or directive transmission line or bus as, for example, a closed-loop or closed-circuit transmission line. A modem will be called a transmitter-receiver as the case may be.
An SS multiplex communication network comprises a transmission line capable of transmitting an SS multiplex signal. The transmission line may be of the form of a closed loop or may have two ends. A plurality of modems are coupled to the transmission line. Generally speaking, the modems carry out transmission and reception of information signals, such as speech, data, and/or picture signals, with the spectra thereof spread by pseudorandom or pseudo noise (usually abbreviated to PN) code sequences. For this purpose, a plurality of PN code sequences may preliminarily be assigned or allotted to the respective modems. Each PN code sequence is a pseudorandom succession of unit pulses, positive and negative, of a considerably high PN code sequence generating clock (briefly, PN clock) rate. A PN code of a relatively large number of such pulses is repeatedly used in the PN code sequence. The period of repetition will herein be named a frame period. When expressed by the number of pulses or bits, the frame period will be referred to as a frame length.
On transmitting a transmission information signal from an originating modem to a destination modem, a PN code sequence is modulated by the information signal into an SS signal and sent through the transmission line. As will later be described more in detail, the PN code sequence used as a modulation carrier is usually the PN code sequence assigned to the destination modem. No severe restriction is imposed on the information signal except that the information signal should have a data clock rate appreciably lower than the PN clock rate as will presently be described if the information signal is a digital signal. Merely by way of example, the information signal may be a PCM (pulse code modulated) signal having a data clock rate of 64 kHz. When the PN and the data clock rates are denoted by f.sub.0 and f.sub.D and the frame length, by N, the restriction is such that the data clock rate f.sub.D should not be higher than the PN clock rate divided by the frame length f.sub.0 /N. A typical PN clock rate is 8.13 MHz for a frame length of one hundred and twenty-seven bits. The PN clock rate may be as high as several hundred megahertzes.
Other modems may concurrently supply SS signals to the transmission line. Inasmuch as such SS signals are multiplexed into an SS multiplex signal carried by different PN code sequences, the SS multiplex communication technique or scheme is often called a code division multiplex communication technique. At the destination modem, the SS multiplex signal is demodulated into a received information signal. Briefly speaking, the demodulation is carried out by correlating the SS multiplex signal to the PN code sequence assigned to the destination modem. The received information signal is a reproduction of the transmission information signal. The modems, which thus carry out SS modulation and demodulation, will be named SS modems.
In a conventional multiplex communication network, such as a TDM (time division multiplex) communication network, a certain number of channels are used in multiplexing information signals. The number of channels is predetermined in consideration of traffic in the network and is usually less than the number of modems accommodated by the network. Even when a call is initiated by a modem towards a destination modem, which is idle, the connection for the call is not established when all channels are busy. In other words, call loss is inevitable.
In an SS multiplex communication network, the transmission line must be capable of transmitting signals of a wide or broad frequency band because the information signals are spectrum spread by the PN code sequences into a frequency band between a substantially zero clock rate and the PN clock rate. The transmission line should therefore be, for example, a coaxial cable or an optical fiber. On the other hand, the SS multiplex signal has a small spectral energy density. Furthermore, the SS multiplex signal is little affected by narrow-band interference and provides a received information signal with an excellent SN (signal-to-noise) ratio. Theoretically, no call loss occurs in an SS multiplex communication network. A conventional SS multiplex communication network insures high secrecy of communication. An SS modem is compact. Power consumption is accordingly little. An SS modem is reliably operable unless the transmission line is closed.
The SS multiplex communication network has, however, been inconvenient when the transmission line forms a closed loop. This is because the information signal carried by a modulation PN code sequence repeatedly circulates through the closed-loop transmission line even after reception at a destination modem. It is therefore urgently required to provide an SS modem to be coupled to a closed-loop transmission line of an SS multiplex communication network.
A typical PN code sequence is a repetition of maximum length sequence (hereafter abbreviated to M sequence) codes. Such a repetition will herein be referred to merely as a maximum length code sequence or, more briefly, as an M sequence. As will later be described with reference to one of nearly forty figures of the accomapnying drawing, an M sequence is generated by an M sequence generator by the use of M sequence or PN code sequence generating clocks in compliance with a generating polynomial. For example, an M sequence code is a succession of fifteen bits of plus and minus unity, such as (+--++-+-++++---). When the frame length is thus fifteen bits long, there are three different M sequence codes generated by common PN code sequence generating clocks in compliance with three different generating polynomials, respectively. It is therefore possible to make an SS multiplex communication network accommodate three SS modems. When the frame length is one hundred and twenty-seven bits long, the number of accommodated SS modems increases to eighteen. The number is still small for practical purposes.
An article was distributed 1979 at a meeting of a technical group of the Institute of Electronics and Communication Engineers of Japan and read by Haruo Ogiwara et al. under the title of "Syuhasu Kakusan ni yoru Kanyusya-kei Syusen Tazyuka Hosiki no Teian" (Technical Report No. SE79-104). The article is available only in Japanese except for the title, which reads in English "Subscriber Network Using Spread Spectrum Technique" according to the authors. In the article, as SS multiplex communication network is disclosed in which the SS modems, such as line concentrators of an exchange network, are assigned with M sequences generated by common PN code sequence generating clocks in compliance with different generating polynomials and cyclically bit shifted. When cyclically bit shifted, an M sequence having a frame length of N bits gives N different M sequences, a zero-bit-shifted M sequence inclusive. It may therefore appear that the number of accommodated SS modems will increase to (127.times.18) when cyclically bit shifted M sequences of one-hundred-and-twenty-seven-bit long M sequence codes are used as the different PN code sequences. As discussed in the article and will briefly described hereinafter, a typical number of SS modems is only three hundred and eighty-one under the circumstances. It is therefore desirable more to increase the number of SS modems accommodated in an SS multiplex communication network.
The reason why it is not practical to accommodate (127.times.18) modems, mainly resides in the fact that interference occurs between channels to a considerable extent. Use of M sequences derived by cyclically bit shifting a single M sequence is preferred as regards the interchannel interference. It is possible to specify such M sequences as M sequences generated by clocks having a predetermined clock period in compliance with a single generating polynomial and with different initial values or conditions, respectively. For PN code sequences, what corresponds to the generating polynomial is theoretically a generating or characteristic function. In practice, each PN code sequence has a certain frame period. It would therefore be feasible depending on the circumstances to use, instead of such M sequences, PN code sequences generated by clocks having a predetermined clock period in compliance with a single generating polynominal and with different initial values. respectively.
The SS modems may deal with information signals of different data clock rates. SS signals are derived with much redundancy when information signals of lower data clock rates are SS modulated by the use of PN code sequences for the highest data clock rate. In other words, it would be possible to increase the number of accommodated SS modems if the PN code sequences are more effectively used.
When cyclically bit shifted PN code sequences are used, it becomes necessary to use a synchronizing signal in establishing frame synchronism among the SS modems. In an SS multiplex communication network comprising a closed-loop transmission line, the frame synchronism must be established as correctly as possible with a simplest possible synchronizer despite circulation of the synchronizing signal through the closed-loop transmission line.
On sending an information signal to a destination SS modem, it is desirable to preliminarily know whether or not the destination SS modem is already busy. It is therefore necessary in practice to carry out carrier sensing of sensing whether the PN code sequence used by the destination modem in modulating an information signal is present or absent in the SS signal reaching the SS modem going to initiate the call. It is also desirable depending on the circumstances to furnish as SS multiplex communication network with a broadcasting facility. Such augmentation of services would render the SS modem bulky and accordingly expensive. Power consumption will increase.
As described before, an SS multiplex signal is correlation detected into received information signals of a high SN ratio. Although no call loss theoretically occurs as pointed out also before, the quality of the reproduced information signals degenerates due to the interchannel interference if a great number of information signals are multiplexed into the SS multiplex signal. Data and/or picture signals generally have a large duty cycle, namely, last a considerably long interval of time. In addition, high transmission performance is mandatory on dealing with data and/or picture signals. It is therefore desirable that an SS multiplex communication network be capable of dealing with data and/or picture signals with excellent transmission performance.